The present invention relates to a mastering apparatus for recording onto a glass master and a method for recording onto a glass master, that is, to positioning a driving mechanism of an optical system such as an objective lens used for converging a laser beam onto a glass master.
The improvement of the performance of computers and peripheral devices, and the spread of the Internet have increased opportunities to process a mass of data such as image data and sound data, and these developments have been accompanied by a need for recording media in which a mass of data can be recorded.
Optical disks have a greater recording capacity and higher portability than magnetic recording media such as hard disks. For this reason, some kinds of optical disks such as CD-ROM have already widely spread, and DVD-ROMs (Digital Versatile Disk-ROMs) having a recording capacity greater than that of CD-ROMs are expected to spread rapidly in the future.
In general, a manufacture of commercial optical disks such as CD-ROMs and DVD-ROMs (hereinafter referred to as "commercial disks") involves the fabrication of a master commercial disk by injection molding on a glass master.
Since the process of injection molding results in deterioration in the pit shape and the like, the glass master must have precision much higher than a master commercial disk. For example, since the track pitch of disks currently marketed as commercial products is 1 .mu.m or less, positioning accuracy along the radial direction is required on the order of nanometers for a recording apparatus to manufacture the glass masters.
FIGS. 1 and 2 show a schematic configuration of a conventional mastering apparatus for recording on the glass master. A recording beam generator 1 shown in FIGS. 1 or 2 irradiates a laser beam L1 for recording information. The laser beam is modulated by an optical unit 2 and is guided to an objective lens 3. The objective lens 3 forms information signals of the laser beam from the optical unit 2 on a glass master 6 made of glass. The glass master 6 is secured to a turn table 7 with the side (recording side) coated with a resist facing upwards, and the rotation of the turn table 7 is controlled by a spindle motor 8.
The movement of the objective lens 3 in a focusing direction (a direction of an optical axis of the objective lens 3) is controlled by a focusing mechanism 4. The focusing mechanism 4 is mounted on a slider 5 which controls the movement of the objective lens 3 in a tracking direction (a direction parallel with a recording surface of the glass master).
When recording data on the glass master 6, the movement of the objective lens 3 is controlled by the slider 5 and focusing mechanism 4 to form the information signals on the resist-coated side of the glass master 6 while rotating the glass master 6. Exposure recording on the glass master 6 follows a spiral path from the inner circumference toward the outer circumference.
In order to record data on the glass master 6 at a predetermined pitch, it is necessary to measure the moving distance of the objective lens 3 in the tracking direction accurately. In the prior art, this has been done by measuring the moving distance of the slider 5 that moves the objective lens 3 in the tracking direction using a laser interferometer 21 as shown in FIG. 1 or a linear scale 22 as shown in FIG. 2.
However, the laser interferometer 21 shown in FIG. 1 measures the relative distance between an object (e.g., a mirror 23 mounted on the slider 5) and the interferometer 21 by projecting light L3 upon the mirror 23 and detecting light reflected therefrom. When the distance between the interferometer 21 and the object 23 becomes long, the distance traveled by the laser beam back and forth therebetween is also long. The inventors have discovered that this has resulted in a problem in that accuracy is reduced because the laser beam becomes susceptive to the influence of the fluctuation of the air (fluctuation of the atmospheric pressure) and the fluctuation of the temperature.
Further, the focusing mechanism 4 includes a voice coil motor for moving the objective lens 3 in the focusing direction, and it is known that an interactive movement due to use of leaf springs to support mechanism 4 occurs on the voice coil motor in the tracking direction (a direction parallel with a recording surface of the glass master). Such an interactive movement cause a displacement of the objective lens 3 in the tracking direction even if the slider 5 is in a correct position.
Similarly, in the mastering apparatus for recording onto the glass master utilizing a linear scale 22 as shown in FIG. 2, the position of the objective lens 3 in the tracking direction can not be properly controlled due to the interactive movement of the voice coil motor as described above even if the slider 5 is in a correct position. In addition, since the linear scale 22 and objective lens 3 are in positions apart from each other, the positioning accuracy of the objective lens 3 can not be improved even if there is no interactive movement. Because Abbe's errors attribute to the rotation (rolling errors, pitching errors and yawing errors) of the slider 5 during a travel.
The inventors have discovered that the conventional mastering apparatus for recording on the glass master has had a problem in that errors occur in the positioning of the objective lens 3 due to errors in the measurement of the slider 5, errors attributable to the interactive movement of the voice coil motor, Abbe's errors and the like. As a result, errors occur in pitches of tracks formed by recording pits on the glass master 6.
The present invention has been made in view of the above-described points, and a purpose of the invention is to provide a mastering apparatus and method for recording onto a glass master in which an optical system such as an objective lens can be accurately positioned.